The present invention relates to an electromagnetic wave energy absorbing material which effectively absorbs and accumulates the energy of the electromagnetic wave such as the sunlight, or the like, as a heat energy therein, for use in a solar energy industry.
In the prior art, a black paint is used for a solar collector and is coated over the surface thereof. However, in this case, the efficiency of the solar collector is not very good, and accordingly another material has been expected.
The characteristics of an electromagnetic wave energy absorbing material to be obtained according to the present invention are as follows.
(1) The very large absorptance .alpha. against a wavelength range (0.3-2.0 .mu.m of the sunlight) of the electromagnetic wave to be absorbed, and
(2) The very small emittance .epsilon. against a wider wavelength range (3-50 .mu.m at 100.degree. C.) than that of the electromagnetic wave, in order to minimize the emission loss of the heat energy absorbed.
The relation among the maximum stagnation temperature T.sub.max, the absorptance .alpha., and the emittance .epsilon. of the electromagnetic wave energy absorbing material is expressed in the following formula. ##EQU1## In this formula (1), a .sigma. means the Stefan-Bolzmann constant and S means an electromagnetic wave energy density. From this formula (1), it is readily understood that, in order to raise the maximum stagnation temperature of the electromagnetic wave energy absorbing material, the absorptance is increased and the emittance is reduced.
In FIG. 1 there is shown a graph showing an absorptance .alpha. with reference to an energy absorption efficiency parameter .alpha./.epsilon. and the maximum stagnation temperature T.sub.max of a conventional plane solar collector having a piece of transparent glass.
In FIG. 1, in case of the electromagnetic wave energy absorbing material having the absorptance .alpha. of 1.0, when the emittance .epsilon. is 1.0, i.e. .alpha./.epsilon.=1, the maximum stagnation temperature is 120.degree. C., and when the emittance .epsilon. is 0.1, i.e. .alpha./.epsilon.=10, the maximum stagnation temperature is 225.degree. C. Its temperature rising rate is approximately 88%. The very large absorptance and the very small emittance are essential to the electromagnetic wave energy absorbing material, as described above.